Discussion

Hydatid disease is caused by the larval stage of the Echinococcus tapeworm, where humans act as an accidental intermediate host after ingesting eggs usually released into the environment by a dog. The hexacanth embryo is released in the duodenum where it passes through the intestinal wall into portal circulation and develops into a cyst within the liver. Once in the liver, cysts grow to 1 cm during the first 6 months and 2-3 cm annually thereafter. Hydatid disease can also involve the lung (15%) and other anatomic locations (10%). The right lobe is most the most frequently involved portion of the liver.

Imaging findings depend on the stage of cyst growth. Calcification is seen radiographically in 20-30% of hydatid cysts. During healing, there is dense calcification of the cyst. Sonographically the cyst wall usually manifests as double echogenic lines separated by a hypoechoic layer. Simple cysts do not show internal echoes, although dependent echogenic foci may be seen due to hydatid sand. CT has a high sensitivity and specificity for hepatic hydatid disease.The typical appearance on CT is a cysticliver mass with high attenuation walls on unenhanced CT, even without wall calcification. Daughter cysts may be located peripherally within the mother cyst. On MRI, hydatid cysts demonstrate water signal intensity, and may have a low signal intensity rim on T2 weighted images.